[CMake][GlobalISel] Turn LLVM_BUILD_GLOBAL_ISEL into an option. NFC.
[CMake][GlobalISel] Turn LLVM_BUILD_GLOBAL_ISEL into an option. NFC.

Previously LLVM_BUILD_GLOBAL_ISEL was a boolean variable and although,
this is strictly identical to an option, it did not convey the
information that the user may set it. Options are here for that.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@276306 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm